bug#63311: 30.0.50; [PATCH] smtpmail-send-it split

[Manuel Giraud via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi Eli,

I'm still struggling with making 'smtpmail-send-it' async.  As I'm using
message.el, I'm trying to make it work here.  But message.el in written
in such way that it is quite hard to make it work with a threaded
'smtpmail-send-it' call.

And then, from a message buffer (with a quite large attachment), I've
just evaluted the following: (make-thread #'message-send)... and
everything went ok:
           - Emacs is still responsive: I could switch to other
             buffers, request the status of a VC handled file.
             
           - When the message is finally sent, the buffer is renamed
             accordingly and tagged as sent via mail.

So my question is this one: instead of making 'smtpmail-send-it'
threaded why not have a threaded version of toplevel commands (such as
'message-send')?  We could hopefully do the same for sendmail.el.  Of
course, we should have it as opt-in behind customs
(message-send-in-background and/or mail-send-in-background?)

As for when something goes wrong and we end up in an 'error' call, maybe
we should fix how 'error' behave inside a thread.  What am I missing?
-- 
Manuel Giraud